Bridging the Gap: Part I

A Discussion with Freeman Dyson

Going to another star is a terribly powerful idea, just as going to the Moon was originally. At some point in human history, there will be a leap across the great void not just to the nearest star but to any star that might be interesting to explore.

Their discussion dovetails to a proposal for sailing on solar wind. Nearly 400 years ago astronomer Johannes Kepler observed comet tails blown by a solar breeze and suggested that vessels might likewise navigate through space using appropriately fashioned sails. It is now widely recognized that sunlight does indeed produce a force which moves comet tails and a large, reflective sail could be a practical means of propelling a spacecraft. In fact, one concept explored by NASA centers is to develop an interstellar probe pushed along by sunlight reflected from an ultra-thin sail. Nearly half a kilometer wide, the delicate solar sail would be unfurled in space. Continuous pressure from sunlight would ultimately accelerate the craft to speeds about five times higher than possible with conventional rockets — without requiring any fuel.

Bruce Murray: In the 1970s, when I was director at the Jet Propulsion Laboratory [JPL] at the same time that Voyager set out on its wonderful journey we seemed to be in a period where there were no limits. And yet I became very aware of a sudden end to this period. I couldn’t foresee a way to push beyond the horizon.

Then, in October 1980, just after Voyager’s Saturn encounter, I organized an informal conference in Pasadena. The question I wanted to address was, how do we eventually go to another star? At that point, the only potentially doable system was some kind of giant sail powered by enormously powerful lasers located in the solar system but not on Earth. This seemed to be within the bounds of physical plausibility, which was pretty exciting.

Now, I’d like to ask you, Freeman, even with all that’s happened in the last 20 years, is that still the conclusion?

Freeman Dyson: I think that’s still true, although there is an alternative way– pellet stream propulsion of voyaging to another star that could be somewhat more economical, though more difficult to do. Instead of shooting at your sail with a laser beam, you shoot at it with pellets. The problem is how to catch the pellets, but in principle, the vehicle could be a lot smaller and more compact, so the system could in fact end up being more economical. It hasn’t really been worked out in detail. But I would say the pellet stream is perhaps just as good a contender as the solar sail.

Bruce: Is the energy needed to collect and capture the pellets less than to collect photons with the sail?

Freeman: The energy isn’t any less. A mass on the order of a ton traveling at half the speed of light takes a lot of energy no matter how you do it. The question is, just what is your efficiency? Although the pellet stream doesn’t in the end use all that much less energy, it’s a lot neater from an engineering point of view. Because the vehicle can be less massive, you save energy simply on the total mass. The problem with the laser sail is that the sail itself weighs so much, and you don’t really want that.

Lou Friedman: What about the really advanced technology sail: some kind of wispy carbon structure, with just a few molecules of aluminum as the reflector? That wouldn’t weigh very much.

Freeman: No, it wouldn’t, but at 30 kilometers [about 20 miles] in diameter it adds up to quite a lot.

The question is, are you using light or are you using microwaves? If you’re using light, then you must have a metal surface of some kind to reflect the light. If you’re using microwaves, you can have a network that can be a lot lighter. In fact, we really don’t know how light that can be.

Bob Forward’s proposal, to make the network out of chicken wire and what he calls Starwisp, involves a really wispy kind of a sail driven by microwaves. That could be a great deal lighter.

Lou: Which would you bet on: light or microwaves?

Freeman: I don’t know. I think it’s foolish to make the choice. In all these technological questions, you have to try everything and find out what works.

Lou: JPL is starting microwave experiments right now. They’ve done some in the laboratory, and they’ve done a light-sail experiment. On our Cosmos 1 mission, we’ll try to pulse the sail with a microwave from the Deep Space Network and see if we can measure the acceleration. If we do that, Freeman, will it be the first interstellar propulsion experiment?

Freeman: Maybe, I don’t know. But, of course, there’s a lot of stuff between here and Alpha Centauri. And I think it’s foolish to think that after you’ve explored the solar system, there’s nothing else interesting until you get to Alpha Centauri.

In fact, there’s a lot of stuff along the way. A fellow named Jack Baggaley in New Zealand is observing meteors with radar a project called AMOR, which stands for Advanced Meteor Orbit Radar. He actually sees stuff arriving here on Earth from Beta Pictoris, which I find very delightful. So we’re already getting interstellar stuff, and it’s being measured and observed.

Beta Pictoris is a star with a huge disc of dust around it. The dust presumably is being thrown around by encounters with planets or other objects. So, before we reach the edge of the solar system, we’ll probably see a lot of interlopers on the journey there– for example, comets and asteroids from Beta Pictoris. It’s wrong to think of all that space as empty. There’s all sorts of interesting stuff going on.

Bruce: Let me ask Lou a question: You’re the only one of the three of us who has written a book about solar sailing. What was your conclusion in the book, and what is your conclusion now about solar sailing to another star?

Lou: My favorite line from my book is "Space is big." When I was writing that in the mid-1980s, I was quite negative on the whole notion of interstellar flight. At that time, I held the view that interstellar travel is to us what the airplane was to Leonardo da Vinci. We could think of it, sketch cartoons about it, but we were centuries away from being able to implement it.

I’m not intimidated by the size of the solar sail– even the idea that the sail has to be hundreds of kilometers or so. If I were, I wouldn’t be doing what I’m doing. The thing that is intimidating to me is the power source.

Freeman: Well, that to me is the easiest part. The Sun is just a wonderful source of power once you get out there. That’s something we more or less understand.

What we don’t understand, of course, is the biology and how to provide for the comfort of human beings in quite different environments. That we don’t know anything about. And that, to me, is far more interesting than the problem of solar power.

Lou: To me, the exciting aspect of solar sailing is the idea of lasers and microwaves. They may change our perspective, so maybe interstellar flight is not so unimaginably far away.

Freeman: Well, I don’t consider 500 years a long time.

Interstellar travel is not unimaginable at all. We’ll be there before we know it. Five hundred years is a very short time in the history of the species.